1. Field of the invention
This invention relates to an optical transformer and more particularly to an optical d.c. voltage transformer which is suitable for preventing a d.c. drift by using both a chopper circuit and a synchronous detector circuit in detecting a d.c. voltage to be measured and also suitable for detecting an output voltage having the same polarity as the d.c. voltage to be measured.
2. Description of the Related Art
The conventional optical voltage measuring apparatus is exemplified by an optical voltage sensor shown in FIG. 8 of the accompanying drawings, generally designated by the numeral 3. The voltage sensor 3 comprises a luminous source 1, a first optical-fiber cable 2, a lens 27, a polarizer 4, a quarter-wave plate 5, an electrooptic medium (hereinafter also called "Pockels cell") such as BGO, a photodetector 7, second and third optical-fiber cables 8, 9, optical/electrical converter circuits 10, 11, and an operation circuit 12.
In operation, constant-intensity light emitted from the luminous source 1 is incident on the first optical-fiber cable 2 and is thereby guided to the sensor 3. As it passes through the sensor 3, the light is modified into parallel light by the lens 27, then into linearly polarized light by the polarizer 4, and further into circularly polarized light by the quarter-wave plate 5, whereupon the light reaches the Pockels cell 6. When a d.c. voltage to be measured is applied to the Pockels cell 6, an anisotropy (composite refraction change) proportional to the applied voltage is produced in the refractive index in the direction of the main axis. Namely, the circularly polarized light receives on the emitting end of the Pockels cell 6 the phase difference expressed by the equation: ##EQU1## where l: the length of Pockels light path, d: the crystal thickness in a voltage applying direction, .lambda.: a wavelength, n.sub.0 : the refractive index of Pockels cell with no voltage applied, .gamma..sub.41 : a Pockels factor, V.sub.0 : the amplitude of voltage applied, and .omega.: the angular frequency of voltage applied.
By the phase difference of the equation (1), the circularly polarized light is modified into elliptically polarized light. This elliptically polarized light is divided into two mutually orthotropic signal components by a photodetector orthotropic to the polarizer and is converted into two optical signals of different light intensities proportional to the long axis and the short axis. Assuming that the outputs of the two intensity-modulated emitting lights after passing through the respective optical/electrical converter circuits 10, 11 are V.MDSD/1.MDNM/, V.MDSD/2.MDNM/, partial light that reaches the polarizer 4 via the lens 27, namely, incident light is I0, and ratio constants are k1, k2, the following equations (2) and (3) are obtained: EQU V1=k1I0(1+sin .GAMMA.) (2) EQU V2=k2I0(1-sin .GAMMA.) (3)
Now assuming that k1=k2 as adjustments are made in some way, the following equation also is obtained: ##EQU2##
In the range in which sin2.GAMMA.&lt;&lt;1, sin2.GAMMA. is nearly equal to 2.GAMMA. so that an output proportional to the phase angle, i.e., an output V proportional to the voltage to be measured can be obtained from the operation circuit 12.
In this detection method, since the equation (4) is used, a value to be measured, even a d.c. voltage, can be detected by chopping, without being influenced by the amount of the incident light I0.
In general, however, in the light transmission system comprising the luminous source 1, the second and third optical-fiber cables 8, 9, the polarizer 4, the quarter-wave plate 5, the pockels cell 6 having electrodes to which voltage to be measured is to be applied, the photodetector 7, the optical/electrical converter circuits 10, 11 and the operation circuit 12, the transmission characteristic of the light amount is changed such as due to the temperature characteristic and the variation with elapse of time. Namely, a d.c. drift phenomenon occurs so that the state assuming k1=k2 is difficult to obtain. If k1.noteq.k2, then the error V1-V2 in the equation (4) will increase sharply so that the error will become large in detecting a d.c. voltage.
For the measurement of a d.c. current, it is converted into a d.c. voltage, and then the d.c. voltage is measured. In this measurement, like the foregoing measurement, since the equation (4) is used, the same disadvantage is obtained.
In addition, the conventional d.c. voltage measurement encounters with the following problems.
Currently ferroelectric crystals such as BGO and LiNbO3 crystals are widely used as Pockels cells. Ferroelectric crystals have been regarded as having an optical linearity to an a.c. voltage so that an output having a modulated light intensity proportional to a voltage to be measured. In the case of d.c. voltage, however, when a Pockels cell composed of, for example, BGO or LiNbO3 is placed in a d.c. electric field or voltage, a ferroelectric crystal's own charging phenomenon occurs. The light output is lowered gently such as due to the charge to the cell surface and the movement of space charges within the Pockels cell so that the measurement will be impossible.
Attempts have been made to improve this prior problem as disclosed in, for example, Japanese Patent Laid-Open Publications Nos. 17170/1984, 116555/1984, 29263/1987 and 123162/1989.
Specifically, Japanese Patent Laid-Open Publication No. 17170/1984 shows a method for detecting a d.c. voltage during no light is irradiated from an another luminous source for photoconductive material, utilizing the change of a specific resistance of a photoconductive material between when the photoconductive material is subjected to irradiatation of light and when the photoconductive material is subjected to no irradiation of light. The photoconductive material to be irradiated by light is composed of CdS, CdSe or PdS, and is disposed between the electrodes to which the d.c. voltage to be measured is to be applied.
In this method of the Japanese publication 17170/1984, in order to prevent the light output from being gradually lowered with time under the influence by the electric charge, the resistance between the electrodes to which the d.c. voltage is to be applied is reduced to about 10.sup.4 .OMEGA.cm by light irradiation, thereby releasing electric charge in the Pockels cell. However, the response time to the change of a d.c. voltage to be measure is long, and therefore, this prior method is not suitable for instrumentation and protection of substation facilities.
According to the technology disclosed in Japanese Patent Laid-Open Publication No. 116555/1984, there is provided second electrodes in order that electrical charge is short-circuited, and a rotary switch located between the second electrodes. When the rotary switch is in the on position, the electrodes, to which the d.c. voltage to be measured is to be applied, are short-circuited by the second electrodes to release electrical charge in the Pockels cell. Then the rotary switch is turned to the off position, whereupon measurement of a d.c. voltage takes place. Although it can be carried out with a simple basic construction, this method also is not suitable for the circuit that requires a transient response speed.
Japanese Laid-Open Publication No. 29263/1988 discloses a concept of measuring an electrostatic field, in the same manner as the case of an a.c. electric field, by pulsating the electrostatic field by a chopper in the form of a slitted rotary blade, which is grounded, without rotating the Pockels cell. However, this method also is not suitable for the circuit that requires a transient response speed.
In each of the foregoing chopper methods, any influence by space charge developed in the Pockels cell can be eliminated, but the response time is long and besides the output voltage is detected in terms of a.c. signal.
According to the technology disclosed in Japanese Patent Laid-Open Publication No. 123162/1989, as shown in FIG. 11 of the accompanying drawings, while CW (continuous-wave) light from a d.c. luminous source 170 is transmitted through a light modulator 140 including a polarizer 55, an electrooptics crystal 54, a phase compensator 56, and a photodetector 57, a voltage from an object 163 to be measured is applied to the light modulator 140 after being pulsated by a chopper circuit 10. The light having an intensity modulated by the pulsated voltage is sent to a sampling high-speed light detector 111 where the light is sampled as digital signal components while d.c. components are removed by a variable band-pass filter built in a lock-in amplifier 15. As a result, high-speed a.c. signal components are detected. The voltage from the object 163 to be measured is applied as a trigger signal to the sampling high-speed light detector 111 via a branch circuit 112, trigger signal generator 113, and delay circuit 14.
U.S. Pat. No. 4,446,425 to Valdmanis et al., as reillustrated here in FIG. 12, while light emitted from a pulsed luminous source 150 is transmitted through a light modulator 140 including a polarizer 55, an electrooptics crystal 54, a phase compensator 56, and a photodetector 57, a voltage from an object 53 to be measured is applied to the light detector 140 after being pulsated by a chopper 51 and a variable delay circuit 52. The light having an intensity modulated by the pulsated voltage is divided into two components by the analyzer 57. Then the two light components are sent to photo detectors 58, 59, respectively, and the d.c. component of the same phase is eliminated by a differential amplifier 60, whereupon the a.c. signal is sent to the lock-in amplifier 61 and then to an averager 62 to obtain an averaged output, which is displayed on a display 63. A lock-in amplifier 61 is operated by the output from the chopper 51 in synchronism with the chopper frequency.
The foregoing arrangements are similar to the present invention in some points and is different therefrom in the object of invention. These prior apparatuses are those for only obtaining the result of voltage detection and does not obtain polarity of voltage, retaining neither the function to protect machines except this apparatus nor the function as a control apparatus.
As a significant feature, the apparatus of the Japanese publication 123162/1989 calls for a sampling high-speed light detector 111. In this apparatus, only half-frequency signals are sampled by the sampling high-speed light detector 111 and by means of gate pulse. Therefore, even if the signal from the object to be measured is d.c., the polarity of the output of the differential amplifier 60 is not identical with that of the voltage passed through the lock-in amplifier 61 so that only positive signals are detected.
In the apparatus of the Valdmanis et al. U.S. Patent of FIG. 12, since the outputs from the photodetectors 58, 59 of the light modulator 140 are expressed by the equations (7) and (8) (described below) and are processed by the differential method, compensation takes place by releasing only the d.c. drift components. However, this U.S. Patent is totally silent about the concept of detecting a d.c. voltage corresponding to the polarity of a d.c. voltage to be measured.
In a substation, the polarity of a d.c. voltage is normally positive, but occasionally becomes negative by accident. Upon such an accident, measures must be taken after detecting the negative polarity; for this purpose, it is currently hoped that a transformer capable of measuring the polarity should be realized.
Consequently, each of the foregoing prior technologies is unsatisfactory for use in an optical d.c. voltage measuring apparatus in which a d.c. voltage to be measured is to be processed for instrumentation and protection of substation facilities and in which a d.c. output signal corresponding to the object d.c. voltage and having a polarity similar thereto is to be measured with the degree of precision within the reference JEC-1201 (on transformers for meters such as protective relays) according to the standards of Electrical Technical Committee, the Institute of Electrical Engineers of Japan.